- Written also: Marsh Willow-herb
- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. Surface runners thread-like, tips with bulb-like overwintering buds.
- Height: 10–70 cm (4–28 in.). Stem unbranched–sparsely erect-branched, round, base glabrous, upper part with arched hairs throughout or in streaks, internodes equally long.
- Flower: Corolla regular, pink–purple–white, 5–8 mm (0.2–0.32 in.) broad; petals 4, with notched tips. Sepals 4, with arched and sparse glandular hairs. Stamens 8. Gynoecium fused, a single carpel, stigma club-like; inferior ovary (under tepals), dense arched hairs throughout, also with glandular hairs. Inflorescence a lax raceme.
- Leaves: Opposite (inflorescence alternate), almost stalkless. Blade narrowly ovate–lanceolate–linear, with tapered base, usually with entire margins, revolute, with arched hairs at least along central vein.
- Fruit: Tubular, 4-valved, 4–8 cm (1.6–3.2 in.) long capsule, clearly longer than stalk. Plumed seeds granular, oblanceolate.
- Habitat: Shores, sea-shores, ditches, ponds, springs, waterside meadows which are prone to flooding, swamps, fens, swamps, banks, wasteland, meadows, lower parts of fell tundra.
- Flowering time: July–August.
No fewer than 18 species of willowherb are established in Finland. Only a few of these are to any extent common, and a large proportion only grow either in the south-western tip of the country or in Lapland. Marsh willowherb is the second most common species after rosebay willowherb and by far the most common narrow-leaved, small-flowered willowherb. It can be found almost anywhere in Finland as long as the habitat is moderately nutritious and especially if it floods occasionally.
Floodwaters can tear off the developing bulb-like overwintering buds from the tips of marsh willowherb’s rhizome. These are buoyant and are able to float on the waters to find new habitats. This kind of asexually produced plant is often stronger and more branched than those that grow from seed, and they have a short rhizomatous part. Marsh willowherb’s appearance changes quite a lot depending on its habitat, but it is also known to have some hereditary forms that have adapted to slightly different habitats.
Other common willowherbs that thrive in wetlands are the North American species American willowherb (E. adenocaulon) and fringed willowherb (E. ciliatum), and northern Finland’s Hornemann’s willowherb (E. hornemannii), can usually be differentiated form marsh willowherb on the basis of their more ovate and toothed leaves. Dahurian willowherb (E. davuricum) and E. laestadii both look similar to marsh willowherb and they grow in Lappish fens and wetlands. Dahurian willowherb is often recognized by its alternate leaves, but they can be opposite at the base, like marsh willowherb. Dahurian willowherb has no basal runners and the flowers’ ovaries have curved hairs only at the base, with glandular hairs further up. E. laestadii is much more difficult to identify than marsh willowherb and for this reason it wasn’t described as a new species until the 1970s.